With an exhaust gas purification system in which a catalytic converter employing a three way catalyst is provided to an exhaust conduit of an engine, the conversion efficiencies for HC, CO, and NOx are all highest when the air/fuel ratio of the fuel mixture in the engine is the stoichiometric air/fuel ratio.
For this reason it is widely practiced to provide an oxygen sensor which detects the oxygen concentration in the engine exhaust, and to perform quick response feedback control of the fuel supply amount to the engine based upon this oxygen concentration, so as to maintain the air/fuel ratio of the fuel mixture in the vicinity of the stoichiometric air/fuel ratio.
In this case the air/fuel ratio feedback control is only performed after the oxygen sensor has been activated, since the air/fuel ratio cannot be accurately detected during the time period from when the engine is started to when the oxygen sensor is activated. Further, it is not possible to attain proper conversion efficiency if the three way catalyst has not yet attained its activation temperature. Accordingly, it is difficult to obtain a desirable exhaust gas purification performance from the three way catalyst in the state when the engine is cold.
In order to cope with this problem, it has been disclosed in Tokkai Hei 5-272394 published by the Japanese Patent Office in 1993 to ensure the operational performance of the engine in the state when the engine is cold before the start of air/fuel ratio feedback control by increase correcting the amount of fuel supplied to the engine, and to speed up the activation of the catalyst by delay correcting the timing of ignition of the fuel mixture in the engine.
However with this method, even when the catalyst has arrived at its activation temperature, since the concentration of oxygen is low due to the fuel amount increase correction, the process of oxidation of HC becomes difficult, and it is not possible to obtain a sufficient result in terms of catalyst activation.